Lithium by itself oxidizes pretty readily in air. I bet it has horrible corrosion resistance and I wonder if it has compatibility issues with standard Alodine and other paints and primers that have an etching component in the chemistry? Definitely worth looking into all performance ratings.
Yep. It looks like it (2090) is off-the-shelf available right now, although in restricted thicknesses (0.040" definitely; smaller sizes looks like not). 2090 seems to be really designed as a near-drop-in-replacement for 7075; moderate corrosion resistance, some notch sensitivity, but a handful lighter and stiffer. 2091 seems more in line with a 2024 replacement with a bit more durability in the standard tempers (if I'm reading the limited available data correctly); but it's not off-the-shelf yet.
Even ignoring additional weight reductions from stiffness increases (since we're pretty often at or near minimum gauge anyway), just the density difference looks sweet for things like spars and control surfaces.